This invention relates to liquid-level gauging.
The invention is more particularly concerned with ultrasonic liquid-level gauging sensors.
Ultrasonic liquid-level sensors utilize the fact that ultrasonic vibrations travel freely in a liquid but are rapidly attenuated in air or other gas. If an ultrasonic transducer is mounted on the base of a liquid reservoir so that it directs energy up towards the liquid/air interface, the energy will be reflected back down to the transducer by this interface. By measuring the time taken between transnfission and reception of an energy pulse, it is possible to measure the distance between the transducer and the liquid/air interface and, from this, the depth of liquid.
It is conunon practice for ultrasonic transducers of this kind to be mounted at the lower end of a tube or still well that extends from the bottom to the top of the liquid reservoir. The tube is open at the bottom so that liquid fills the tube to the same depth as in the reservoir outside the tube. The tube serves several purposes. It helps isolate the transducer from other sensors or sources of interference. It also confines the ultrasonic beam, so that it is directed only at the region of the liquid surface directly above the transducer. Furthermore, the tube produces within it a region of liquid surface that is substantially damped of waves.
Another advantage arising out of the use of the tube is that it is easy to provide a reference height, by mounting some form of reflector at a known height within the tube. In this way, the transducer will receive a reflection from the liquid surface and one from the reference reflector against which the liquid height can be calibrated. This enables the ultrasonic gauging system to compensate for different liquids having different acoustic propagation properties and for temperature variations, which can affect ultra-sound propagation. Examples of ultrasonic probes having a tube of this kind are described in, for example, EP 0106677, GB 9304579 and GB 9304561.
There are various problems with existing ultrasonic liquid gauging sensors. One problem arises from the fact that the amplitude of energy reflected back to the sensor varies considerably with change in the angle of the liquid surface relative to the axis of the probe, such as caused by change in attitude of the probe. At angles exceeding about 20 degrees from the vertical, the signal return from the liquid surface can be below the lowest signal-to-noise ratio that is acceptable for reliable measurement of liquid height. In practice, the maximum working angle for presently available probes is about 30 degrees. Even at certain angles considerably less than this there can be blind spots where the return signal drops to a very low level.
Another problem with ultrasonic level sensors is that they are not suitable for use with some liquids, such as engine oil, because of the high attenuation of ultrasonic energy in these liquids. Although it is possible to measure the height of such liquids in a perfectly level, stationary tank, the slightest tilt or perturbation of the liquid surface increases the scattering and absorption, causing the return signal to drop below a usable level.